Most fence posts were made from round timber, hence the shape. This barbaric devise was used specifically for birds of prey whose unfortunate nature is to alight on poles, posts etc to observe and ambush prey. A natural habit that was exploited by trappers for centuries.

This type of trap was usually called a pole trap, in the past I released a few birds of prey such as harriers and buzzards from this vicious trap that gamekeepers set on the fence posts then neglected to return and check.